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8/2/2019 Switching 97

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What is it all about?

How do we move traffic from one part of the network to another?

Connect end-systems to switches, and switches to each other

Data arriving to an input port of a switch have to be moved toone or more of the output ports


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Types of switching elements

Telephone switches

switch samples

Datagram routers

switch datagrams

ATM switches

switch ATM cells


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Classification

Packet vs. circuit switches

packets have headers and samples dont

Connectionless vs. connection oriented

connection oriented switches need a call setup

setup is handled in control planeby switch controller

connectionless switches deal with self-containeddatagrams

Connectionless

(router)

Connection-oriented

(switching system)Packetswitch

Internet router ATM switching system

Circuitswitch

Telephone switchingsystem


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Other switching element functions

Participate in routing algorithms

to build routing tables

Resolve contention for output trunks

scheduling

Admission control

to guarantee resources to certain streams

Well discuss these later

Here we focus on pure data movement


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Requirements

Capacity of switch is the maximum rate at which it can moveinformation, assuming all data paths are simultaneously active

Primary goal: maximize capacity

subject to cost and reliability constraints

Circuit switch must reject call if cant find a path for samples

from input to output

goal: minimize call blocking

Packet switch must reject a packet if it cant find a buffer to store

it awaiting access to output trunk goal: minimize packet loss

Dont reorderpackets


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A generic switch


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Outline

Circuit switching

Packet switching

Switch generations

Switch fabrics

Buffer placement

Multicast switches


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Circuit switching

Moving 8-bit samples from an input port to an output port

Recall that samples have no headers

Destination of sample depends on timeat which it arrives at theswitch

actually, relative order within a frame

Well first study something simpler than a switch: a multiplexor


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Multiplexors and demultiplexors

Most trunks time division multiplex voice samples

At a central office, trunk is demultiplexed and distributed toactive circuits

Synchronous multiplexor

N input lines

Output runs N times as fast as input


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More on multiplexing

Demultiplexor

one input line and N outputs that run N times slower

samples are placed in output buffer in round robin order

Neither multiplexor nor demultiplexor needs addressing

information (why?)

Can cascade multiplexors

need a standard

example: DS hierarchy in the US and Japan


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Inverse multiplexing

Takes a high bit-rate stream and scatters it across multipletrunks

At the other end, combines multiple streams

resequencing to accommodate variation in delays

Allows high-speed virtual links using existing technology


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A circuit switch

A switch that can handle N calls has N logical inputs and Nlogical outputs

N up to 200,000

In practice, input trunks are multiplexed

example: DS3 trunk carries 672 simultaneous calls

Multiplexed trunks carry frames= set of samples

Goal: extract samples from frame, and depending on position inframe, switch to output

each incoming sample has to get to the right output line and theright slot in the output frame

demultiplex, switch, multiplex


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Call blocking

Cant find a path from input to output

Internal blocking

slot in output frame exists, but no path

Output blocking

no slot in output frame is available

Output blocking is reduced in transitswitches

need to put a sample in one of severalslots going to the desirednext hop


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Time division switching

Key idea: when demultiplexing, position in frame determinesoutput trunk

Time division switching interchanges sample position within aframe: time slot interchange (TSI)


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How large a TSI can we build?

Limit is time taken to read and write to memory

For 120,000 circuits

need to read and write memory once every 125 microseconds

each operation takes around 0.5 ns => impossible with current

technology

Need to look to other techniques


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Space division switching

Each sample takes a different path thoguh the swithc,depending on its destination


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Crossbar

Simplest possible space-division switch

Crosspointscan be turned on or off

For multiplexed inputs, need a switching schedule(why?)

Internally nonblocking but need N2 crosspoints

time taken to set each crosspoint grows quadratically

vulnerable to single faults (why?)


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Multistage crossbar

In a crossbar during each switching time only one crosspoint perrow or column is active

Can save crosspoints if a crosspoint can attach to more thanone input line (why?)

This is done in a multistage crossbar

Need to rearrange connections every switching time


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Multistage crossbar

Can suffer internal blocking

unless sufficient number of second-level stages

Number of crosspoints < N2

Finding a path from input to output requires a depth-first-search

Scales better than crossbar, but still not too well

120,000 call switch needs ~250 million crosspoints


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Time-space switching

Precede each input trunk in a crossbar with a TSI

Delay samples so that they arrive at the right time for the spacedivision switchs schedule


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Time-space-time (TST) switching

Allowed to flip samples both on input and output trunk

Gives more flexibility => lowers call blocking probability


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Outline

Circuit switching

Packet switching

Switch generations

Switch fabrics

Buffer placement

Multicast switches


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Packet switching

In a circuit switch, path of a sample is determined at time ofconnection establishment

No need for a sample header--position in frame is enough

In a packet switch, packets carry a destination field

Need to look up destination port on-the-fly

Datagram

lookup based on entire destination address

Cell

lookup based on VCI

Other than that, very similar


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Repeaters, bridges, routers, and gateways

Repeaters: at physical level

Bridges: at datalink level (based on MAC addresses) (L2)

discover attached stations by listening

Routers: at network level (L3)

participate in routing protocols

Application level gateways: at application level (L7)

treat entire network as a single hop

e.g mail gateways and transcoders

Gain functionality at the expense of forwarding speed

for best performance, push functionality as low as possible


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Port mappers

Look up output port based on destination address

Easy for VCI: just use a table

Harder for datagrams:

need to find longest prefix match e.g. packet with address 128.32.1.20

entries: (128.32.*, 3), (128.32.1.*, 4), (128.32.1.20, 2)

A standard solution: trie


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Tries

Two ways to improve performance

cache recently used addresses in a CAM move common entries up to a higher level (match longer strings)


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Blocking in packet switches

Can have both internal and output blocking

Internal

no path to output

Output

trunk unavailable

Unlike a circuit switch, cannot predict if packets will block (why?)

If packet is blocked, must either buffer or drop it


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Dealing with blocking

Overprovisioning

internal links much faster than inputs

Buffers

at input or output

Backpressure

if switch fabric doesnt have buffers, prevent packet from entering

until path is available

Parallel switch fabrics

increases effective switching capacity


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Outline

Circuit switching

Packet switching

Switch generations

Switch fabrics

Buffer placement

Multicast switches


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Three generations of packet switches

Different trade-offs between cost and performance

Represent evolution in switching capacity, rather than in

technology

With same technology, a later generation switch achieves greater

capacity, but at greater cost

All three generations are represented in current products


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First generation switch

Most Ethernet switches and cheap packet routers

Bottleneck can be CPU, host-adaptor or I/O bus, depending


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Example

First generation router built with 133 MHz Pentium

Mean packet size 500 bytes

Interrupt takes 10 microseconds, word access take 50 ns

Per-packet processing time takes 200 instructions = 1.504 s

Copy loopregister


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Second generation switch

Port mapping intelligence in line cards

ATM switch guarantees hit in lookup cache

Ipsilon IP switching assume underlying ATM network

by default, assemble packets

if detect a flow, ask upstream to send on a particular VCI, andinstall entry in port mapper => implicit signaling


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Third generation switches

Bottleneck in second generation switch is the bus (or ring)

Third generation switch provides parallel paths (fabric)


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Third generation (contd.)

Features

self-routing fabric

output buffer is a point of contention

unless we arbitrateaccess to fabric

potential for unlimited scaling, as long as we can resolve contentionfor output buffer


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Outline

Circuit switching

Packet switching

Switch generations

Switch fabrics

Buffer placement

Multicast switches


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Switch fabrics

Transfer data from input to output, ignoring scheduling andbuffering

Usually consist of links and switching elements


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Crossbar

Simplest switch fabric

think of it as 2N buses in parallel

Used here for packetrouting: crosspoint is left open longenough to transfer a packet from an input to an output

For fixed-size packets and known arrival pattern, can computeschedule in advance

Otherwise, need to compute a schedule on-the-fly (what doesthe schedule depend on?)


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Buffered crossbar

What happens if packets at two inputs both want to go to sameoutput?

Can defer one at an input buffer

Or, buffer crosspoints


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Broadcast

Packets are tagged with output port #

Each output matches tags

Need to match N addresses in parallel at each output Useful only for small switches, or as a stage in a large switch


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Switch fabric element

Can build complicated fabrics from a simple element

Routing rule: if 0, send packet to upper output, else to loweroutput

If both packets to same output, buffer or drop


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Features of fabrics built with switching elements

NxN switch with bxb elements has elements withelements per stage

Fabric is self routing

Recursive

Can be synchronous or asynchronous

Regular and suitable for VLSI implementation

logbN N b/


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Banyan

Simplest self-routing recursive fabric

(why does it work?) What if two packets both want to go to the same output?

output blocking


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Blocking

Can avoid with a buffered banyan switch

but this is too expensive

hard to achieve zero loss even with buffers

Instead, can check if path is available before sending packet

three-phase scheme

send requests

inform winners

send packets

Or, use several banyan fabrics in parallel

intentionally misroute and tag one of a colliding pair

divert tagged packets to a second banyan, and so on to k stages

expensive

can reorder packets

output buffers have to run k times faster than input


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Sorting

Can avoid blocking by choosing order in which packets appearat input ports

If we can

present packets at inputs sorted by output

remove duplicates remove gaps

precede banyan with a perfect shuffle stage

then no internal blocking

For example, [X, 010, 010, X, 011, X, X, X] -(sort)->

[010, 011, 011, X, X, X, X, X] -(remove dups)->[010, 011, X, X, X, X, X, X] -(shuffle)->[010, X, 011, X, X, X, X, X]

Need sort, shuffle, and trap networks


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Sorting

Build sorters from merge networks

Assume we can merge two sorted lists

Sort pairwise, merge, recurse


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Merging


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Putting it together- Batcher Banyan

What about trapped duplicates?

recirculate to beginning or run output of trap to multiple banyans (dilation)


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Effect of packet size on switching fabrics

A major motivation for small fixed packet size in ATM is ease ofbuilding large parallel fabrics

In general, smaller size => more per-packet overhead, but morepreemption points/sec

At high speeds, overhead dominates! Fixed size packets helps build synchronous switch

But we could fragment at entry and reassemble at exit

Or build an asynchronous fabric

Thus, variable size doesnt hurt too much

Maybe Internet routers can be almost as cost-effective as ATMswitches


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Outline

Circuit switching

Packet switching

Switch generations

Switch fabrics

Buffer placement

Multicast switches


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Buffering

All packet switches need buffers to match input rate to servicerate

or cause heavy packet loses

Where should we place buffers?

input in the fabric

output

shared


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Input buffering (input queueing)

No speedup in buffers or trunks (unlike output queued switch)

Needs arbiter Problem: head of line blocking

with randomly distributed packets, utilization at most 58.6%

worse with hot spots


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Dealing with HOL blocking

Per-output queues at inputs

Arbiter must choose one of the input ports for each output port

How to select?

Parallel Iterated Matching

inputs tell arbiter which outputs they are interested in

output selects one of the inputs

some inputs may get more than one grant, others may get none

if >1 grant, input picks one at random, and tells output

losing inputs and outputs try again Used in DEC Autonet 2 switch


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Output queueing

Dont suffer from head-of-line blocking

But output buffers need to run much faster than trunk speed(why?)

Can reduce some of the cost by using the knockoutprinciple

unlikely that all N inputs will have packets for the same output

drop extra packets, fairly distributing losses among inputs


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Shared memory

Route only the header to output port

Bottleneck is time taken to read and write multiported memory

Doesnt scale to large switches But can form an element in a multistage switch


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Datapath: clever shared memory design

Reduces read/write cost by doing wide reads and writes

1.2 Gbps switch for $50 parts cost


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Buffered fabric

Buffers in each switch element

Pros

Speed up is only as much as fan-in

Hardware backpressure reduces buffer requirements

Cons

costly (unless using single-chip switches)

scheduling is hard


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Hybrid solutions

Buffers at more than one point

Becomes hard to analyze and manage

But common in practice


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Outline

Circuit switching

Packet switching

Switch generations

Switch fabrics

Buffer placement Multicast switches


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Multicasting

Useful to do this in hardware

Assume portmapper knows list of outputs

Incoming packet must be copied to these output ports

Two subproblems

generating and distributing copes

VCI translation for the copies


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Generating and distributing copies

Either implicit or explicit

Implicit

suitable for bus-based, ring-based, crossbar, or broadcast switches

multiple outputs enabled after placing packet on shared bus

used in Paris and Datapath switches

Explicit

need to copy a packet at switch elements

use a copynetwork

place # of copies in tag

element copies to both outputs and decrements count on one ofthem

collect copies at outputs

Both schemes increase blocking probability


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Header translation

Normally, in-VCI to out-VCI translation can be done either atinput or output

With multicasting, translation easier at output port (why?)

Use separate port mapping and translation tables

Input maps a VCI to a set of output ports

Output port swaps VCI

Need to do two lookups per packet

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